1. Field of the Invention
The present invention relates to a copying machine using an electrophotographic technology, LBP and other image formation system, and the multicolored image formation system with two or more colors.
2. Related Background Art
FIG. 2 is a block diagram for the image forming system, whereby the conventional environmental control and its correction method are explained. Here described is a case where the present invention is used for a laser beam printer, that is, the mechanism of forming images by scanning a laser beam on the photosensitive drum in synchronization with reading of an original will be explained below.
First, an original 9 is read by a CCD 1. The obtained analog image signal is amplified to a given level by an amplifier 2, and then converted into an 8-bit digital image signal (0 to 255 gradation) by a A/D converter 3. Next, the digital image signal passes through a gamma (.gamma.) converter 10 (gradation control table containing a 256 type RAM) for gamma correction, and then enters a D/A converter 14.
The digital signal is converted into an analog signal again. Then, the comparator 16 compares the analog signal with a signal of a specific frequency which is generated by a triangular wave generator 15. Then, the pulse width is modulated. The binary coded image signal whose pulse width has been modulated enters the laser drive circuit 17 as it is, which is used as an emission ON/OFF control signal for the laser diode 18. A laser beam emitted from the laser diode 18 is scanned in main scanning direction by a known polygonal mirror 19. Then, after passing through the f/.theta. lens 20 and reflection mirror 21, the beam is irradiated on the photosensitive drum 22 or an image supporting or bearing material which is rotating in direction shown by arrow to thereby form an electrostatic latent image.
On the other hand, the photosensitive drum 22 is uniformly discharged by the exposure unit 28, and charged uniformly with negative electricity by the electrostatic charger 23. After that, when the photosensitive drum receives a laser beam mentioned above, it forms an electrostatic latent image on its surface according to the image signal. In addition, the so-called image scanning method or a method of exposing the portion to be developed (black pixels) is employed as usually so in the laser beam printer. Therefore, the developing unit 24 uses the known reverse development method to adhere toner with a negative charge characteristic to the portion of the photosensitive drum 22 discharged by laser. Thus, the latent image becomes visible.
FIG. 3 shows the relations between the surface potential of the photosensitive drum and the development contrast when the said reverse development is performed. Here, V.sub.D represents the negative potential charged uniformly by the charger 23 shown in FIG. 2. V.sub.OO represents the potential obtained when the laser diode is driven with the image signal of OO.sub.H (.theta. level) which has been digitized. Potential on the surface of the photosensitive drum V.sub.FF is the potential obtained in the same way as mentioned above for FF.sub.H (256 levels). Therefore, assuming that .vertline.V.sub.DEV -V.sub.FF .vertline. as shown in FIG. 3 is contrast potential Vcont and the development density developed with Vcont is Dmax, the Vcont should be set appropriately to optimize image density (generally, approximately 1.2 to 1.8 in electrophotography). (In general, Vcont may be .vertline.V.sub.D -V.sub.FF .vertline.).
It should be noted that the background removing potential (Vback) in FIG. 3 is used to fully remove the fog or background from a white-ground portion of an image which is irradiated with light quantity OO.sub.H.
The visible image formed on the photosensitive drum 22 according to the procedure mentioned above (a toner image with negative charge) is transferred to a recording material (paper, in general) 26. The remaining toner on the photosensitive drum 22 is scraped off by the cleaner 27 later. Then, said series of processes is repeated again.
For a laser beam printer with the configuration mentioned above, the conventional environmental control varies the contrast potential mentioned previously (Vcont) depending on the environment so that an optimal image density can be output constantly. That is to say, as shown in FIG. 4, if the development characteristic (V-D curve) varies with the environment as A for high humidity, B for normal humidity, and C for low humidity, Vcont is changed to Vcont-a for high humidity, Vcont-b formal humidity, and Vcont-c follow humidity as shown in FIG. 5. Thus, Dmax will be constant at 1.5 regardless of the environment.
To realize the environmental control, a temperature/humidity sensor 11 is provided as shown in FIG. 2. According to the absolute humidity detected and the Vcont table of the solid lines in FIG. 5 that is stored in memory 13, the CPU 12 calculates a proper Vcont value to change the amount of charge in the charger 23.
The environmental dependency of the V-D curve shown in FIG. 4 varies with the humidity adjustment state of developer in the developing unit. The temperature/humidity sensor 11 is, therefore positioned near the developing unit 24 so that the humidity adjustment state of developer can be well reflected on the sensor. However, even with such control, Dmax dependent on the environment is not always constant at 1.5 due to the machine type, the deterioration of the developer or minor difference in the production lot of developer. Namely, Dmax may become constant at 1.6 or 1.4.
To correct these variations, a correction means that adds or subtracts a certain amount of correction to or from the previously mentioned Vcont in all environments has been devised. Namely, a means to shift the Vcont table in FIG. 5 vertically by parallel movement has been made available. This correction has been provided, for instance, as a correction means used when the V-D curve changes in any environment from A, B, or C to A', B', or C' as shown in FIG. 6 along with the deterioration of developer or difference in the production lot number, or as a correction means used when the aimed Dmax cannot be obtained due to the distance between the development sleeve and photosensitive drum, or the difference in the amount of developer on the sleeve.
However, as mentioned above, such a way of correction that a certain amount of correction is added (or subtracted) to the function shown in FIG. 5 has drawback of spoiling the stability of Dmax achieved by the said environmental control.
To be more specific, if the V-D curve indicating development characteristic is changed from A, B, or C to A', B', or C' because of the deterioration of developer or delicate differences in the characteristics of developer for a machine having environmental variations shown in FIG. 4, or if a machine is produced in the way that it will have a characteristic represented with the V-D curve of A', B', or C' in FIG. 6 instead of the standard V-D curves in FIG. 4, constant Dmax of 1.5 cannot be output with the Vcont table in FIG. 5 intact. Therefore, the table must be corrected to return Dmax to 1.5. For instance, Vcont is added by .DELTA.V to characteristic C' in FIG. 6 so as to obtain 1.5 of Dmax, .DELTA.V of Vcont is also added to A' and B'. Then, the Vcont table is modified to be the dotted line in FIG. 5. As a result, Dmax is not the same in three environments any longer. Namely, Dmax-A'&gt;Dmax-B'&gt;Dmax-C'=1.5. This means that the employment of conventional correction will spoil environmental control for stabilization of density.
This is also true when correction is made to return Dmax to 1.5 based on the V-D curve of A'. In this case, 1.5=Dmax-A'&gt;Dmax-B'&gt;Dmax-C'. This is because the same amount of correction is applied for all environments although the amount of contrast potential necessary to change Dmax differs from environment to environment. Consequently, the conventional correction method has drawback that it is poor at following up the environment.